Energy wise I haven't got a clue to be honest. But chocolate supposedly produces serotonin -the "happy hormone". when you eat it so supposedly it makes u feel happier because of this. (This being the main reason we can feel like we're craving it.)
I had a bit a couple of days ago (oh the shame) and I actually felt awful after it, not happy at all as a result. I'm not sure if that was the chocolate itself or the guilt, though.